Salmonella is a major pathogen of poultry, and its presence in poultry flocks is a major concern for human public health. Incidents of food poisoning caused by Salmonella remain prevalent throughout the world, and are second only to Campylobacter as the most common cause of food poisoning due to bacteria. Not surprisingly, poultry products are a primary source of Salmonella infections in humans, with the transfer of this pathogen originating either from laying hens to eggs, or from broiler carcasses to meat products. Salmonella Enteritidis (SE) is especially important in egg-associated food poisoning due to the ability of some strains to colonize the reproductive tissues of hens, which can result in the presence of live Salmonella in the eggs and/or on the egg shell.
Contamination by Salmonella can be controlled to an extent through the use of effective biosecurity measures, competitive exclusion products, and disinfection procedures. These measures are used in conjunction with vaccination to provide the optimum protection against a poultry flock becoming contaminated, with vaccination being an effective aid in reducing the contamination of eggs by SE. Indeed, since the introduction of vaccines in Salmonella control programs in the 1990's, their use has become widespread throughout many parts of the world.
The inactivated vaccines NOBILIS SALENVAC® and SALENVAC T®, which were introduced first, have been almost entirely replaced in the egg producer market by live vaccines, due to the early protection afforded and reduced cost, particularly in labour, as administration can be accomplished through the drinking water. Thus within the egg producing portion of the poultry industry, vaccination with a live Salmonella vaccine during the first week of life has become standard practice in the United Kingdom and is becoming more common throughout the European Union. Whereas repeated vaccination is claimed to offer protection of the hens throughout the flock life, many producers follow an initial live vaccination program with a dose or course of an inactivated vaccine. The basis of this latter procedure is that antibodies present in the yolks of eggs laid by hens that had been vaccinated with an inactivated vaccine have been shown to reduce the growth of Salmonella compared to those from either unvaccinated hens or hens vaccinated with a live vaccine. Inactivated vaccines also remain common practice in the poultry meat industry, as they provide passive protection to newly hatched chicks against Salmonella via transfer of antibodies through the egg [Inoue et al. Avian Diseases 52:567-571 (2008)].
Salmonella spp. is a Gram-negative bacterial genus that is divided into two species, S. enterica and S. bongori. S. enterica contains 6 major serogroups (A, B, C1, C2-3, D, and E) that have been classified according to their lipopolysaccharide structure. Essentially all existing vaccines on the market claim to protect against Salmonellae of serogroups B (i.e., serovar S. Typhimurium) and D (i.e., serovar S. Enteritidis) and thereby, specifically target the reduction of these serovars. Indeed, until recently, these serogroups were not only the most prevalent, but also the most important from a zoonotic viewpoint. However, though S. Enteritidis and S. Typhimurium are still the most common serovars involved in human disease in Europe, their prevalence in contaminated chicken products appears to be declining, whereas Salmonellae of the serogroups C1 (which includes e.g., S. Infantis, S. Mbandaka, and S. Virchow) and C2-3 (which includes, e.g., S. Hadar, S. Newport and S. Kentucky) have significantly increased in prevalence. Notably, a vaccine that comprises serovars S. Typhimurium (serogroup B) and S. Enteritidis (serogroup D) has been shown to provide cross-protection against other serovars of serogroup B, i.e., S. Heidelberg and S. Agona, but failed to show any efficacy against a challenge with S. Hadar (serogroup C2-3), indicating a need for additional antigens in vaccines to provide the necessary protection.
More recently, an inactivated trivalent vaccine containing S. Enteritidis (serogroup D), S. Typhimurium (serogroup B), and S. Infantis (serogroup C1), has been reported to be efficacious against a challenge from S. Enteritidis, S. Typhimurium, S. Infantis, or S. Heidelberg (serogroup B) [Deguchi et al., Avian Disease 53:281-286 (2009)]. In addition, another inactivated trivalent vaccine containing S. Typhimurium (serogroup B), S. Mbandaka (serogroup C1) and S. Orion (serogroup E) also has been reported to protect against several serogroup B and C1 serovars, though the results with the serogroup E serovars were inconclusive [Pavic et al., Avian Pathology 39 (1):31-39 (2010)].
The O antigen is a major lipopolysaccharide component of the cell surface of all Gram-negative bacteria. Each of the serogroups of S. enterica express distinguishable O antigens and accordingly, the O antigen can be used in their classification [Nori and Thong, African Journal of Microbiology Research 4(9) 871-876 (2010)]. Consistently, little to no cross-reaction was found between the six different S. enterica serogroups when an enzyme-linked immunosorbent assay was performed (ELISA) [Smith et al., J. Vet. Diagn. Invest., 7:481-487 (1995)].
The O antigen is encoded by multiple genes in the rib gene cluster of the S. enterica genome and not surprisingly, there are significant differences within the rfb gene clusters between the 6 major serogroups of S. enterica [Lee et al., J. Gen. Microbiol., 138:1843-1855 (1992); Nori and Thong, African Journal of Microbiology Research 4(9) 871-876 (2010)]. More particularly, the rfb gene cluster of serogroup C1 has been shown to have only limited similarity to the rfb cluster of serogroups A, B, C2-3, D, or E [Lee et al., J. Gen. Microbiol., 138:1843-1855 (1992)]. Furthermore, a monoclonal antibody raised against a heated alcohol-acetone-extracted serogroup C2-3 serovar (S. Newport) reacted with protein-free lipopolysaccharides from other serogroup C2-3 serovars, but not with protein-free lipopolysaccharides from any other serogroup. In addition, the reactivity with this monoclonal antibody was inhibited by preincubation of the corresponding antigen from serogroup C2-3 with polyclonal rabbit anti-serogroup C2-C3 antibodies, but not by preincubation of that antigen with polyclonal antisera obtained from serogroup C1 antigen, or any other Salmonella serogroup antigen tested [Duffey et al., J. Clin. Microbiol. 30(12):3050-3057 (1992)].
Although vaccines against specific Salmonella serogroups have been commercially available for several decades, there remains a need to provide Salmonella vaccines that can protect against both Salmonella serogroups C1 and C2-3, as well as against serogroups B and D.
The citation of any reference herein should not be construed as an admission that such reference is available as “prior art” to the instant application.